PROJECT SUMMARY Cancer immunotherapy has made great strides in recent years, yet improved approaches are required to achieve more durable responses in a greater number of patients. Blockade of the inhibitory receptor programmed-death 1 (PD-1) enables tumor-reactive T cells to effectively recognize and eliminate tumors in a subset of responsive patients, but many patients are resistant to PD-1 blockade. To improve response rates, we need to better understand the mechanisms that lead to therapeutic resistance. The overall goal of this project is to understand the role of T cell?intrinsic factors that contribute to patient response versus resistance to PD-1 blockade. We will identify signaling pathways and gene-expression patterns associated with blockade response with a particular focus on the role of T-cell receptor (TCR) affinity. We hypothesize that the make-up of TCR affinities of tumor-specific T cells affects tumor antigen recognition, activation signaling, and downstream gene expression, thus contributing to patient response to therapy. Analysis of these parameters in patient samples and mouse models of PD-1 blockade will allow us to identify T cell properties characteristic of response to therapy. In our first aim we will isolate and quantify the TCR-binding properties of melanoma antigen-specific T cells from PD-1 blockade?responsive and ?resistant patients to determine how TCR affinity profiles influence PD-1 blockade responses. In our second aim we will use proteomic and genomic analysis of patient antigen-specific T cells to develop a global signature of PD-1 blockade response and resistance. In our third aim we will use a panel of melanoma patient?derived TCRs with varying affinities for the same antigen for in vitro and in vivo experiments to determine how TCR affinity influences response to PD-1 blockade. Overall, we will determine the contribution of TCR affinities of melanoma-specific T cells to blockade resistance and will provide evidence to support the targeting of specific T cells based on TCR affinity for combination therapies and the selection of patients likely to respond to PD-1 blockade therapy based on TCR affinity profiles.